1. Field of the Invention
The present invention relates to an eye refractometer or an examining and testing instrument such as retinoscope and eye fundus camera.
2. Description of the Prior Art
To effect a precise measurement or observation, and also to obtain a good photographic picture, it is essentially of importance to correctly align and space the instrument with and from the eye. If alignment is incorrectly made, then there will be caused an error of measurement. In the case of a fundus camera, a portion of the picture taking light may be dispersed by the iris of the eye due to the incorrect alignment. Also, incorrect spacing causes various problems. Usually, spacing is made by structurally predetermining the distance between the objective lens of the instrument and a holder on which the face of the examinee is unmovably held. The adjustment of the distance between the objective lens and the eye, that is generally called the working distance, is very important in particular when a fundus camera is used. If the operator fails to accurately adjust the working distance, a portion of light illuminating the fundus is reflected by the cornea of the eye and the reflected light adversely enters into the picture taking light. As a result, a flare is produced in the picture image. When a correct spacing is achieved, it will also contribute to a substantial improvement of measuring accuracy of an instrument such as a refractometer.
U.S. Pat. No. 3,871,772 describes a positioning system of an eye examining instrument. According to the teaching of this prior art, the anterior part of eye to be examined is uniformly illuminated with infrared light and the eye anterior is observed by employing an aiming device supported on the body of the apparatus. Alignment and spacing are adjusted by aligning the center of the aiming plate with the center of the image of the eye pupil. However, this prior art system has some drawbacks. The whole of the eye anterior becomes visible in the visual field of the aiming device. Furthermore, the resolving power of the image is substantially reduced due to the presence of an image intensifier mounted in the aiming device. Therefore, it is impossible or difficult to judge whether or not the image of the eye anterior is clear and sharp. For this reason, the adjustment of both alignment and spacing must be carried out in accordance with the deviation of the center of the image of the pupil from the center of the aiming plate. Alignment and spacing can not be carried out independently of each other.
For a conventional fundus camera, alignment is usually carried out in the following manner:
Initially, the camera body is moved in the direction away from the eye so as to focus the objective lens on the pupil of eye. Then, the position of the camera is adjusted in vertical and horizontal directions until the center of the view field in the view finder coincides with the center of the pupil. After this adjustment, the camera is moved back to the first position.
Thus, alignment is relatively easy to carry out, but, the above described procedure is not applicable for spacing. It is, therefore, very difficult to carry out spacing independently of alignment.